A sea monster in the sky

SkyCaramba astronomy blog for the week ending April 21, 2012

The biggest constellation is a sea monster. Hydra is so long, when its head rises it takes five more hours for the tip of its tail to be above the horizon with it.

The old legends say Hercules killed Hydra. But the Hydra of ancient Greece and Rome wasn’t a one-headed snake. It had many heads. And cutting one off caused two to grow in its place. Defeating Hydra was such a hard job, any job thought to be very hard today can be called a Herculean task.

I have never heard anyone talk about a Herculean solution but the ancient hero killed this sea monster by cauterizing each stump of the snake with a torch before it could sprout two more heads. One of the heads wouldn’t die, so he put it under a heavy stone so it couldn’t bother anyone. In case you’re wondering, Hercules is also a constellation. It’s a ways to the northeast of Hydra.

Another legend says Hydra was the son of Typhon, a dreaded giant who fought the gods. Another story says the serpent was Typhon himself. Typhon chased the gods from Olympus into Egypt. But the gods got the upper hand when they learned how to make thunderbolts. Zeus struck and killed Typhon and the gods could return to Olympus.


The only named star in Hydra is Alphard. The name means the solitary one. Indeed, Alphard looks a bit lonely. It’s the only bright star in that section of the sky. It’s a challenge to see all the other stars that make the serpent’s body in light polluted skies.


Alphard has an orange glow. It’s 40 times as big as our sun and would be 400 times as bright from the same distance.

It’s a barium star. In other words, lots of barium is created in the stellar processes active in Alphard. It’s thought that it had a companion that died long ago and some of the companion’s material sloughed off into Alphard.

Unlike the Hydra of the old legends, the constellation has just one head. It’s the northernmost part of the constellation. You could see it just a little above the horizon at the north pole. Hydra’s northernmost star is 6° north of the celestial equator. That means you could see it almost all the way to the south pole too.

The southernmost part of the constellation is not quite all the way to Hydra’s tail. You can see it as far north as about 50° and all the way south to the pole.

Hydra straddles the meridian, the imaginary line over your head from north to south, around mid-evening in mid-April. Try to make out all the stars. ¡SkyCaramba!